1. Field of the Invention
This invention is an improvement for rack and pinion gear sets wherein the rack shaft is supported by a bearing that allows axial displacement and some rocking motion. More particularly, the invention pertains to the nature of the surface supporting the rack shaft and the position of its center with respect to the locus of the resultant forces applied by the pinion to the teeth of the rack shaft.
2. Description of the Prior Art
In conventional rack and pinion steering gear systems, both ends of the rack shaft extend outwardly from a steering gear housing and are connected to tie rods that transmit steering forces to the wheels of the vehicle. The ends of the rack shaft and tie rods are subject to road loading which applies severe impact and vibratory loads to the steering system. In order to avoid premature failure, the bending strength of the shaft must be sufficient to sustain the dynamic loading.
It is conventional practice, to cut the teeth of the rack shaft only to a shallow depth of the cross section of the shaft in order that the residual portion of the cross section will have sufficient strength to withstand the dynamic load environment in which the steering system must operate. Although this technique produces satisfactory results regarding the bending strength of the shaft, it has been recognized that the loads applied to the rack shaft by the pinion operate to produce an undesirable rotation of the rack shaft about its central longitudinal axis. This phenomenon is the result of the rack teeth being cut to a shallow depth which necessarily locates the resultant of the applied forces at a position substantially eccentric of the axis of the shaft. The loads have a tangential component about the axis which tends to rotate the shaft and cause the teeth of the rack to rotate, at one extremity of the tooth length, into closer meshing engagement with the pinion. The teeth at the opposite extremity of the tooth length, rotate out of meshing engagement with the pinion. As a result, greater contact force develops at the end where engagement is increased and lesser contact results at the opposite end of the tooth. Due to the nonuniform distribution of the loads across the tooth length, the bending stress in the tooth is substantially greater at the root of the tooth adjacent the increased contact loading zone. Of course, at the opposite end of the tooth, the average bending stresses at the root are less than if the contact load were uniformly distributed along the tooth length. It has been calculated that the increase in bending stress at the most highly loaded extremity of the tooth exceeds the average bending stress of the tooth by approximately 20% as compared with the bending stresses that would result if the load were uniformly distributed.
One approach taken in the prior art to reduce rotation of the shaft and to more uniformly distribute the contact loads between the pinion and the rack has been to cut the teeth deeply into the rack cross-section so that the resultant of the forces applied by the pinion aligns with the axis of the shaft. When this is done, rotation of the shaft is theoretically avoided and the contact forces and associated stresses are more uniformly distributed. However, the cross-section of the rack shaft that must transmit the bending forces is substantially reduced by this machining technique and the ability of the shaft to withstand bending loads in the dynamic environment is substantially reduced.